18 BYZANTINE CHURCHES CHAP. (p. 198) seems to have had only four windows from the beginning. The ribs of a Byzantine dome are not constructive In the same way as are the ribs of a Gothic vault. They were built along with the rest of the dome and of the • same material, and are in no way separate from the infilling, though they no doubt strengthened the shell of the dome by their form.1 On the outside a circular shaft with a very simple cap Is often placed at the angles of the piers, and from these shafts the brick cornice springs in a series of arches over the windows. Sometimes the angle Is formed by a point between two half-shafts, as in the domes of the narthex in S. Theodore (p. 246). External Treatment.—In the older churches the exterior seems to have been left in simple masses of brickwork, impressive only by their size and proportion. Probably even this effect was not considered of great importance. In later times a very beautiful system of decoration with slender shallow niches was introduced and was applied In particular to the east end and to the apses. The finest examples of this system on a large scale are seen at the Pantokrator (p. 235) and S. Theodosia (p. 173). Carefully considered or elaborate external compositions are rare, and the only examples In Constantinople are the side chapel of the Pammakaristos (p. 154) and the narthex of S. Theodore (p. 246). External Marble and Mosaic.—Marble and mosaic, we have reason to know, were occasionally used on the exterior of churches,2 though no fragments remain. On the south side of the Pantepoptes (p. 216) the string-course does not correspond to the line of the walls, but projects in a manner which shows that marble must have been employed to line the large windows* A similar projection of the string-course or cornice is not uncommon elsewhere, though not so evident as in the Pantepoptes, and may have been made to receive a marble or mosaic lining. 1 See p. 154. 2 Dome of the Rock at Jerusalem. S. Mary Peribleptos ; see Vlda de Gran Tamorlan y Itlnerario del Ruy Gonzalez de Clawijo, p, 52.